I have seen several posts about the NLRB issuing a complaint based on an employee's facebook post about their supervisor, that led to additional comments by fellow employees. See Nolo's Employment Law Blog's post as an example, NLRB Sues Employer for Firing Employee Over Facebook Post.

When I talk about social media, I always make the point that although the technology is new, the principles that need to be applied already exist. The law about concerted activity is quite developed; shoot, it was well developed when I had my first 5th Circuit argument back in 1979. There the issue was a comment made at a meeting about an announcement that the employer was going to require everyone to take two weeks off as they moved the printing plant where the employees worked. It's that same body of law that now gets dusted off, because instead of a comment made at a worker's meeting, it is a comment issued through a new mode of communication.

Although it is not to say that there is nothing ever new, it is true that it is probably more rare than we think.